Condensate gas pressure-temperature phase diagram calculation method and application thereof

Abstract

The invention belongs to the technical field of oil-gas field development engineering, and particularly relates to a condensate gas pressure-temperature phase diagram calculation method and application thereof. The method comprises the following steps: (1) solving fluid component data of a condensate gas system through a cubic state equation and calculating a mixture component fugacity coefficient to obtain a condensate gas phase diagram; (2) determining a maximum pressure characteristic point and a maximum temperature characteristic point of the condensate gas phase diagram by adopting a phase stability analysis algorithm and combining a dichotomy; (3) dividing the whole pressure and temperature plane according to the maximum pressure feature point and the maximum temperature feature point; and (4) calculating phase boundaries of the divided regions by adopting a dichotomy in combination with a phase stability analysis algorithm, and integrating data points to obtain a P-T phase diagram. The method is high in calculation speed and reliable in operation, compared with a traditional saturated pressure curve tracking method and a bubble point line and dew point line calculation method, solving of a nonlinear equation set is avoided, and the stability of a pressure and temperature phase diagram algorithm is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas field development engineering, and in particular relates to a condensate gas pressure-temperature phase diagram calculation method and application thereof. Background Art

[0002] The calculation of the pressure-temperature phase diagram of the oil and gas hydrocarbon system is crucial to the development of condensate gas reservoirs. According to the pressure and temperature of the gas reservoir, the phase state of the formation fluid in the oil and gas reservoir (gas, liquid or gas-liquid coexistence) can be known from the phase diagram, and the permeability of the formation fluid can be predicted in combination with the relative permeability curve. Especially for condensate gas reservoirs, once the pressure drops below the saturation pressure of the condensate gas system during the pressure depletion process, retrograde condensation will occur in the formation, that is, the phenomenon of condensate oil being produced in the pores of the formation rock due to the pressure drop. Retrograde condensation is a manifestation of reservoir damage. The phase diagram can be used to determine the timing and extent of retrograde condensation damage in condensate gas reservoirs.

[0003] The calculation of the pressure-temperature phase diagram of oil and gas hydrocarbon systems has always been a difficult and hot topic in the field of reservoir fluid phase state research. There are currently two mainstream calculation methods: 1) saturation pressure curve tracking method; 2) separate calculation of bubble point line and dew point line.

[0004] 1) Saturation pressure curve tracking method.

[0005] Assuming that the oil and gas hydrocarbon fluid mixture contains N components, the independent variables of the entire system are:

[0006]

[0007] The N+2 independent variables in equation (1) can be solved by specifying one of the variables and then solving the N+1 variables at the same time. A certain point on the pressure-temperature phase diagram of the oil and gas hydrocarbon system is a set of solutions represented by equation (1) that satisfies a specific gas-liquid ratio (100% liquid phase on the bubble point line and 100% gas phase on the dew point line). Michelsen et al. used the equilibrium constant K to represent all the molar numbers of the unknown quantities. i After substitution, the calculation starts from a very low pressure condition. After calculating two points on the saturated pressure curve, the previous results are used for interpolation as the initial value of the next point on the saturated pressure curve. The entire algorithm uses Newton's method to solve, showing that when the pressure changes from low to high, the algorithm traverses the entire saturated pressure curve. If all goes well, the phase diagram of the oil and gas hydrocarbon system can be quickly solved. When using the saturated pressure curve tracking algorithm, since the Newton method needs to be used to solve N+1 unknowns, it must require N+1 equations. The equation group solved by this method when calculating the hydrocarbon oil and gas system phase diagram is:

[0008]

[0009] In formula (2), and are the fugacity of gas and liquid phase respectively. There are N equations in this system of equations, where N is the number of components in the fluid;

[0010]

[0011] Formula (3) is the gas-liquid phase material conservation equation, where z i For the total composition of the fluid group, K i is the equilibrium constant, and F is the gasification fraction (the mole fraction of the gas phase). i ) and gasification fraction (F) are specified, the above N+1 equations contain N+1 variables, and there are three cases:

[0012] α=(lnK1,lnK2,…,lnK N ,lnT) T #(4)

[0013] α=(lnK1,lnK2,…,lnK N ,lnP) T #(5)

[0014] α=(lnK1,lnK2,…,lnK N ,lnP,lnT) T #(6)

[0015] The first case (Formula (4)) allows the calculation of the saturation temperature and all equilibrium constants by a given pressure; the second case (Formula (5)) allows the calculation of the corresponding saturation pressure and all equilibrium constants by a given temperature; the third case (Formula (6)) allows the specification of any equilibrium constant K i , thereby calculating the saturation pressure, saturation temperature and the remaining N-1 equilibrium constants at the same time. Therefore, the calculation of the phase diagram is a combination of the above three situations. It is necessary to realize the reasonable switching of the solution variables in the three cases near some key positions (such as the maximum pressure point, maximum temperature point, and critical point of the phase diagram) to calculate the complete phase diagram. Therefore, its technical details (initial value of the multivariable Newton method, switching of solution variables and its judgment, etc.) are difficult to implement, and the calculation is not very stable. Even if the algorithm has been integrated into mainstream commercial software, it often fails to converge in actual calculations.

[0016] 2) Calculate the bubble point line and dew point line separately.

[0017] This method establishes the solution equations for the bubble point line and the dew point line respectively. By solving the bubble point line and the dew point line respectively, a complete pressure-temperature phase diagram is obtained. The critical point of the two-component system is obtained through the intersection of the bubble point line and the dew point line, that is, at this point, the gas phase and the liquid phase cannot be separated. At a fixed temperature, the bubble point pressure of the mixture system is the pressure at which the first bubble appears in the pure liquid phase system. Similarly, the dew point pressure is the pressure at which the first drop of liquid appears in the gas phase. The trajectory of these points forms a phase envelope, and the algorithms for the dew point and bubble point are very close.

[0018] For a gas-liquid equilibrium system, the vaporization fraction (f rg ) can be solved by the Rachford-Rice equation:

[0019]

[0020] When f rg When it approaches 0, it represents the bubble point. According to the definition of equilibrium constant:

[0021]

[0022] At the bubble point, the liquid phase composition (x i ) and the total component (z i ) is very close, so we have:

[0023]

[0024] Similarly, when f rg When it approaches 1, it represents the dew point.

[0025]

[0026] The above two equations are extremely important in the calculation of dew point and bubble point pressure. When solving the bubble point pressure curve, it is necessary to obtain the temperature of the first bubble obtained by isothermal decompression of the mixed system from the liquid phase. Converted to the minimum pressure of the liquid phase at a fixed temperature, this bubble point pressure is obtained by solving the N+1-dimensional nonlinear equation group. The control equation group is the N+1 equations represented by equations (2) and (9). The N+1th equation is obtained by equation (9) and can be expressed as:

[0027]

[0028] In the process of solving the dew point pressure, the N+1th equation is obtained from equation (10), which can be expressed as:

[0029]

[0030] It can be seen that whether using the saturated pressure curve tracking algorithm or the method of calculating the bubble point line and dew point line separately, it is necessary to solve a complex set of nonlinear equations. The calculation reliability is largely dependent on the correctness of the algorithm implementation, and more seriously depends on the algorithm implementer's mastery of this part of theoretical knowledge.

[0031] Therefore, a new method for determining the saturation pressure curve based on phase stability analysis is needed, which can be realized without solving complex nonlinear equations. Currently, no method has been found in the prior art to combine stability analysis with dichotomy to solve the condensate gas phase diagram. Summary of the invention

[0032] In view of the above shortcomings, the purpose of the present invention is to provide a method for calculating the pressure-temperature phase diagram of condensate gas. The method is based on the phase stability analysis theory and can accurately calculate the phase state of condensate gas reservoirs during production. By understanding the changes in its phase state, the timing of the occurrence of formation retrograde condensation and the changes in the degree of retrograde condensation with production time can be predicted.

[0033] To achieve the above object, the technical solution adopted by the present invention is:

[0034] The present invention aims to provide a method for calculating a condensate gas phase diagram based on the phase stability analysis theory of oil and gas hydrocarbon systems. The advantage of this method is that in the process of calculating the phase diagram of the condensate gas mixture system, a continuous iteration method can be selected to solve the phase stability analysis problem and then draw the phase diagram, thereby avoiding the numerical stability problem caused by using the Newton method to solve the nonlinear equation group, and the calculation speed is fast, which is more concise and easy to implement than the saturation pressure curve tracking method and the direct solution of the bubble point line and the dew point line.

[0035] The prerequisites of this method are: 1) obtaining the fluid component data of the condensate gas system; 2) a reliable and stable method for solving the cubic state equation and calculating the fugacity coefficient of the mixture components; 3) a reliable stability analysis algorithm.

[0036] A method for calculating a condensate gas pressure-temperature phase diagram, the method comprising:

[0037] (1) Fluid component data of the condensate gas system is obtained by solving the cubic state equation and calculating the fugacity coefficient of the mixture components to obtain the condensate gas phase diagram;

[0038] (2) The maximum pressure characteristic point and the maximum temperature characteristic point of the condensate gas phase diagram are determined by using a phase stability analysis algorithm combined with a dichotomy method;

[0039] (3) Dividing the entire pressure-temperature plane according to the maximum pressure characteristic point and the maximum temperature characteristic point;

[0040] (4) The phase boundaries of the divided areas are calculated using the bisection method combined with the phase stability analysis algorithm, and the data points are integrated to obtain the PT phase diagram.

[0041] Preferably, the process of acquiring the fluid component data of the condensate gas system in step (1) includes: a. performing component tests on the oil and gas samples separated on the ground; b. estimating the components of the condensate gas in the formation based on the production gas-oil ratio during stable production and the surface oil and gas component test.

[0042] Preferably, the search process for the maximum pressure characteristic point in step (2) includes: in the temperature range of 0-500°C, the interval is 45-55°C, preferably 50°C; under each temperature condition, the saturation pressure value is obtained by using a stability analysis algorithm combined with a dichotomy method, and in the two temperature intervals with the largest saturation pressure value, the temperature interval is reduced by 9-11 times, preferably 10 times; continue to use the same method to obtain the saturation pressure, and repeat this step until the algorithm converges to Pmax.

[0043] Further preferably, the convergence condition is 1e-5MPa.

[0044] Preferably, the search process for the maximum temperature characteristic point in step (2) includes: between 0.1MPa-Pmax with a pressure interval of 2-4MPa, preferably an interval of 3MPa; at each pressure, along the temperature axis, a stability analysis and a dichotomy method are used to calculate the saturation temperature value at the corresponding pressure, and within the two pressure intervals with the largest saturation temperature values, the pressure interval is reduced by 9-11 times, preferably 10 times; the same method is continued to be used to calculate the saturation temperature; and this step is repeated until the algorithm converges to Tmax.

[0045] Further preferably, the convergence condition is 1e-3°C.

[0046] Preferably, the division step in step (2) includes: using the temperature value corresponding to the maximum pressure characteristic point to draw a vertical line on the temperature coordinate axis, and using the pressure value corresponding to the maximum temperature characteristic point to draw a vertical line on the pressure coordinate axis, and the two vertical lines divide the entire pressure-temperature plane into 4 areas.

[0047] Preferably, the calculation process of the phase boundary in step (4) comprises the following steps:

[0048] S1. Select the region. Select a point in the gas-liquid two-phase region as point P1, and select a point in the single-phase region as point P2;

[0049] S2, stability analysis is performed at the low pressure point P1 and the high pressure point P2 respectively, and the results returned by the stability analysis are unstable and stable respectively;

[0050] S3. Calculate the average value P of P1 and P2m , in P m If P m The stability analysis at the point returned a stable result, using P m Instead of the upper pressure boundary P2 used in the initial calculation; if P m The stability analysis at the point returned an unstable result, using P m Replace the lower pressure boundary P1 used in the initial calculation;

[0051] S4. Repeat steps S1-S3, and combine stability analysis and dichotomy to calculate the saturation pressure value at the temperature, and obtain the phase boundaries in each pressure-temperature region.

[0052] Further preferably, the region in step S1 includes three regions, and when the saturation pressure is calculated, the temperature range of region 1 is 0-Tpmax, wherein Tpmax is the temperature corresponding to the maximum pressure point Pmax, and the temperature ranges of regions 2 and 3 are both Tpmax-Tmax.

[0053] Further preferably, it is characterized in that the initial calculated pressure range of region 1 is Ptmax-70MPa, the initial calculated pressure range of region 2 is Ptmax-70MPa, and the initial calculated pressure range of region 3 is 0.1MPa-Ptmax.

[0054] Specifically, the present invention proposes for the first time a phase diagram drawing method for locking the saturation pressure boundary by combining a stability analysis algorithm with a dichotomy method, specifically referring to Figure 3 As shown, at a certain temperature in region 1, the stability analysis is first performed at point P1. Since the pressure at point P1 is low enough and is in the gas-liquid two-phase region of the phase diagram, the result returned by the stability analysis is unstable (two-phase). Then, the stability analysis is performed at point P2. Since the pressure at point P2 is high enough and is in the single-phase region, the result returned by the stability analysis is stable (single-phase). Therefore, the saturation pressure curve (the boundary between single-phase and two-phase) must be between points P1 and P2. At this time, the average value between P1 and P2 is calculated using formula (13):

[0055]

[0056] Then at the average pressure point P m Then conduct stability analysis. If P m The stability analysis result at the pressure P is stable. m At this point, the fluid is still in the single-phase region, and P m Instead of the upper pressure boundary P2 used in the initial calculation, it can be seen that the upper pressure boundary phase saturation pressure shrinks; if P m The stability analysis result at the position is unstable, indicating that Pm The point is located in the gas-liquid two-phase region of the fluid, and P m Instead of the pressure lower boundary P1 used in the initial calculation, it can be seen that the liquid phase saturation pressure at the pressure lower boundary shrinks. m After replacing the upper or lower pressure boundary, the saturation pressure value at the given temperature can be quickly obtained by repeatedly using formula (13) and the above process in combination with stability analysis and dichotomy.

[0057] In region 1, change the temperature and repeat the above process to obtain a section of saturated pressure boundary in region 1. All saturated pressure boundaries can be found by using the same method in regions 2 and 3. In region 1, when calculating the saturated pressure, the temperature range is 0 to Tpmax, where Tpmax is the temperature corresponding to the maximum pressure point Pmax, and the lower limit of the initial value of the dichotomy pressure is Ptmax and the upper limit is 70MPa (the upper limit of the saturated pressure of condensate gas is generally lower than this pressure value); when calculating the saturated pressure in regions 2 and 3, the temperature range is Tpmax to Tmax, but the pressure range is slightly different. The initial calculated pressure range in region 2 is Ptmax (unstable) to 70MPa (stable), while the initial calculated pressure range in region three is 0.1MPa (stable) to Ptmax (unstable).

[0058] The boundary points determined in regions 1, 2, and 3 are connected together to form a complete pressure-temperature phase diagram of the condensate gas.

[0059] The present invention also relates to the application of the above-mentioned condensate gas pressure-temperature phase diagram calculation method in the development of condensate gas reservoirs.

[0060] Compared with the prior art, the present invention has the following beneficial effects:

[0061] (1) The method for calculating the condensate gas pressure-temperature phase diagram based on the phase stability analysis theory provided by the present invention comprises the following steps: using a phase stability analysis algorithm combined with a dichotomy method to determine two characteristic points of the condensate gas phase diagram, namely, the maximum pressure and the maximum temperature; dividing the entire pressure-temperature plane into four regions according to the temperature corresponding to the maximum pressure point and the pressure corresponding to the maximum temperature point; selecting three of the regions and using a dichotomy method combined with a phase stability analysis algorithm to draw phase boundaries respectively, and finally completing the calculation of the entire phase diagram.

[0062] (2) This method has fast calculation speed and reliable operation. Compared with the traditional saturation pressure curve tracking method and bubble point line and dew point line calculation method, it avoids solving nonlinear equations and improves the stability of the pressure-temperature phase diagram algorithm. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 The pressure-temperature phase diagram of the volatile fluid in the oil and gas reservoir of Example 1;

[0064] Figure 2 The result diagram of the stability analysis algorithm in Example 1 after implementing the stability analysis in the temperature and pressure range of 0-500 degrees Celsius and 0.1-70 MPa with a temperature step of 1 degree Celsius and a pressure step of 0.1 MPa;

[0065] Figure 3 The algorithm proposed in Example 1 is Figure 2 A schematic diagram of the phase boundary is drawn within the range of region 1; Figure 3 Points P2 and P1 represent the upper and lower pressure boundaries at a certain temperature during the calculation process;

[0066] Figure 4 The algorithm proposed in Example 1 is Figure 2 A schematic diagram of the phase boundary is drawn within the range of region 2;

[0067] Figure 5 The algorithm proposed in Example 1 is Figure 2 A schematic diagram of the phase boundary is drawn within the range of region 3;

[0068] Figure 6 It is the algorithm flow chart of the present invention;

[0069] Figure 7 A condensate gas phase diagram drawn in Example 2 based on the method proposed by the present invention;

[0070] Figure 8 This is the effect diagram after local encryption of the condensate gas phase diagram at the maximum temperature point (Tmax) in Example 3;

[0071] Fig. 9 This is a phase diagram drawn using the saturation pressure direct calculation algorithm in Comparative Example 1;

[0072] Fig.10 This is the phase diagram calculated using the saturation pressure curve tracing method in Comparative Example 2. DETAILED DESCRIPTION

[0073] The technical solutions in the embodiments of the present invention are further clearly described. The described embodiments are only a part of the present invention and are used to explain the present invention but not to limit the present invention. Therefore, other embodiments obtained by other technicians in the field without creative work all belong to the protection scope of the present invention.

[0074] Example 1

[0075] The oil and gas samples separated on the ground are tested for components respectively; the components of condensate gas in the formation are estimated based on the gas-oil ratio during stable production and combined with the surface oil and gas component test; the condensate gas phase diagram of a certain place is obtained through phase stability analysis and calculation, such as Figure 1 shown. Figure 1 It is obtained by performing phase stability analysis on the entire pressure-temperature region, which requires large amounts of calculation and is time-consuming. The following example illustrates how to directly obtain the phase boundary between the gas and liquid phases through phase stability analysis, that is, directly draw a phase diagram.

[0076] Calculation method of condensate gas pressure-temperature phase diagram:

[0077] Step 1: Compile a stable and reliable stability analysis algorithm for oil and gas hydrocarbon systems. The specific implementation of this part refers to Michelsen ML The isothermal flash problem. PART I. Stability. Fluid Phase Equilibria, 1982 (9): 1-19.

[0078] Step 2: Use the stability analysis algorithm to search for the maximum pressure (Pmax) and maximum temperature (Tmax) characteristic points on the pressure-temperature phase diagram.

[0079] Step 3: Divide the entire pressure-temperature plane into four regions by the temperature corresponding to Pmax and the pressure corresponding to Tmax, and select three of them ( Figure 2 When dividing the pressure-temperature plane, the temperature value corresponding to the maximum pressure characteristic point is used to draw a vertical line on the temperature coordinate axis, and the pressure value corresponding to the maximum temperature characteristic point is used to draw a vertical line on the pressure coordinate axis. The two vertical lines divide the entire pressure-temperature plane into 4 regions (see Figure 2 ).

[0080] Figure 2The results of stability analysis performed by the stability analysis algorithm in the temperature step of 1 degree Celsius and the pressure step of 0.1MPa in the temperature and pressure ranges of 0-500 degrees Celsius and 0.1-70MPa show the overall characteristics of the phase diagram of the target condensate fluid. The shaded area in the figure is the single-phase unsaturated fluid area, and the white area is the gas-liquid two-phase coexistence area. Although the figure depicts the pressure-temperature boundary of the condensate gas phase diagram (the transition area between the single phase and the gas-liquid two-phase), it took about 350,000 stability analysis calculations to draw the figure, which is relatively time-consuming (the calculation time using MATLAB programming is about 1 hour, and the C++ language is about 9 minutes). In order to reduce unnecessary calculations, the maximum pressure point (Pmax) and the maximum temperature point (Tmax) are first determined using a pressure step of 3MPa and a temperature step of 50℃, and then the gas-liquid two-phase area is divided into 3 regions according to the temperature corresponding to the maximum pressure point and the pressure corresponding to the maximum temperature point ( Figure 2 1, 2, and 3 in the figure) to prepare for the subsequent search for the saturation pressure curve in each area.

[0081] Step 4: Figure 2 In region 1, the phase boundary is calculated by using the bisection method combined with the stability analysis algorithm ( Figure 3 ). In this area, the calculation proceeds from low temperature to high temperature. The low temperature starts from 0 degrees Celsius and the calculation stops at the temperature corresponding to Pmax. The temperature interval or step size during this period can be set by the density of points in the phase diagram. Generally, a phase diagram drawn with 50 points in one area can fully meet the actual needs.

[0082] Figure 3 Points P2 and P1 represent the upper and lower pressure boundaries at a certain temperature during the calculation process. Pressure P1 is in the gas-liquid two-phase region of the fluid, and the result returned by the phase stability analysis is unstable; pressure P2 is in the single-phase region of the fluid, and the result returned by the phase stability analysis is stable. Therefore, when programming calculations, stability can be represented by 1 and instability can be represented by 0. Therefore, the upper and lower pressure boundaries can be updated using the dichotomy method until P1 and P2 finally converge to the saturation pressure curve.

[0083] Step 5: Figure 2 In region 2, the phase boundary is calculated by using the bisection method combined with the stability analysis algorithm ( Figure 4 ). In this region, the calculation proceeds from low temperature to high temperature. The low temperature starts from the temperature corresponding to Pmax and the calculation stops at Tmax. In this step, as the temperature approaches Tmax, the phase diagram As it approaches infinity, the phase diagram calculation points around Tmax can be locally encrypted.

[0084] Step 6: Figure 2 In region 3, the phase boundary is calculated by using the bisection method combined with the stability analysis algorithm ( Figure 5 ). In this region, the calculation is performed from low temperature to high temperature. The rest is the same as step 4.

[0085] Step 7: Integrate the data points to get the complete phase diagram.

[0086] comprehensive Figures 2 to 5 The calculation method shown in the figure and the above specific implementation details, the present invention can realize the rapid drawing of the condensate gas pressure-temperature phase diagram. The overall implementation flow chart of the present invention is as follows: Figure 6 shown.

[0087] Example 2

[0088] The phase diagram of a condensate gas reservoir was calculated using the steps in Example 1, and a program was written using MATLAB. The boundaries of all regions in the phase diagram were accurately calculated ( Figure 7 ), the overall running time is 0.2897 seconds. Figure 7 The vertical line in the middle is the temperature corresponding to the maximum pressure point, and the horizontal line is the pressure corresponding to the maximum temperature point. The two lines divide the entire pressure-temperature range into four areas ( Figure 7 ). In these four regions, the phase boundary in the high-pressure and low-temperature region (upper left) is represented by an inverted triangle symbol, the phase boundary in the high-pressure and high-temperature region (upper right) is represented by a circle symbol, and the phase boundary in the low-pressure and high-temperature region (lower right) is represented by a square symbol, indicating that the phase boundary is calculated by the method of the present invention. During the entire phase diagram calculation process, there was no algorithm crash or uncalculated pressure and temperature conditions, and the calculation effect was stable and reliable.

[0089] Example 3

[0090] The relatively sparse points at the maximum pressure point in Example 2 were locally encrypted ( Figure 8 ), calculation time 0.2908 seconds.

[0091] Comparative Example 1

[0092] Fig. 9 The traditional method of calculating the bubble point line and the dew point line separately is adopted. Compared with the method proposed in the present invention, this method cannot converge at the critical point and the calculation time is very long, which is about 3.3693 seconds.

[0093] Comparative Example 2

[0094] Fig.10 The calculation result of the saturation pressure curve tracking method is compared with the method proposed in the present invention. This method has a faster calculation speed, but poor stability, and is prone to algorithm errors during the calculation process. The calculation is unstable.

[0095] The above detailed description is a specific description of one feasible embodiment of the present invention. The embodiment is not intended to limit the patent scope of the present invention. Any equivalent implementation or modification that does not deviate from the present invention should be included in the scope of the technical solution of the present invention.

Claims

1. A method for calculating a condensate gas pressure-temperature phase diagram, characterized in that: The condensate gas pressure-temperature phase diagram calculation method comprises: (1) Fluid component data of the condensate gas system is obtained by solving the cubic state equation and calculating the fugacity coefficient of the mixture components to obtain the condensate gas phase diagram; (2) The maximum pressure characteristic point and the maximum temperature characteristic point of the condensate gas phase diagram are determined by using a phase stability analysis algorithm combined with a dichotomy method; (3) Dividing the entire pressure-temperature plane according to the maximum pressure characteristic point and the maximum temperature characteristic point; (4) The phase boundaries of the divided areas are calculated using the bisection method combined with the phase stability analysis algorithm, and the data points are integrated to obtain the PT phase diagram.

2. The method for calculating the condensate gas pressure-temperature phase diagram according to claim 1, characterized in that: The process of acquiring the fluid component data of the condensate gas system in step (1) includes: a. performing component tests on the oil and gas samples separated on the ground; b. estimating the components of the condensate gas in the formation based on the production gas-oil ratio during stable production and the surface oil and gas component test.

3. The method for calculating the condensate gas pressure-temperature phase diagram according to claim 1, characterized in that: The search process for the maximum pressure characteristic point in step (2) includes: in the temperature range of 0-500°C, at intervals of 45-55°C, using a stability analysis algorithm combined with a binary method to calculate the saturation pressure value under each temperature condition; in the two temperature intervals with the largest saturation pressure values, reducing the temperature interval by 9-11 times, and continuing to use the same method to calculate the saturation pressure, and repeating this step until the algorithm converges to Pmax.

4. The method for calculating the condensate gas pressure-temperature phase diagram according to claim 3, characterized in that: The convergence condition is 1e-5MPa.

5. The method for calculating the condensate gas pressure-temperature phase diagram according to claim 1, characterized in that: The search process for the maximum temperature characteristic point in step (2) includes: using a pressure interval of 4-5 MPa between 0.1 MPa and Pmax, and using a combination of stability analysis and dichotomy along the temperature axis at each pressure to calculate the saturation temperature value at the corresponding pressure; in the two pressure intervals with the largest saturation temperature values, reducing the pressure interval by 9-11 times, and continuing to use the same method to calculate the saturation temperature; repeating this step until the algorithm converges to Tmax.

6. The method for calculating the condensate gas pressure-temperature phase diagram according to claim 5, characterized in that: The convergence condition is 1e-3°C.

7. The method for calculating the condensate gas pressure-temperature phase diagram according to claim 1, characterized in that: The division step described in step (2) includes: using the temperature value corresponding to the maximum pressure characteristic point to draw a vertical line on the temperature coordinate axis, and using the pressure value corresponding to the maximum temperature characteristic point to draw a vertical line on the pressure coordinate axis, and the two vertical lines divide the entire pressure-temperature plane into 4 areas.

8. The method for calculating the condensate gas pressure-temperature phase diagram according to claim 1, characterized in that: The calculation process of the phase boundary in step (4) includes the following steps: S1. Select the region. Select a point in the gas-liquid two-phase region as point P1, and select a point in the single-phase region as point P2; S2, stability analysis is performed at the low pressure point P1 and the high pressure point P2 respectively, and the results returned by the stability analysis are unstable and stable respectively; S3. Calculate the average value P of P1 and P2 m , in P m If P m The stability analysis at the point returned a stable result, using P m Instead of the upper pressure boundary P2 used in the initial calculation; if P m The stability analysis at the point returned an unstable result, using P m Replace the lower pressure boundary P1 used in the initial calculation; S4. Repeat steps S1-S3, and combine stability analysis and dichotomy to calculate the saturation pressure value at the temperature, and obtain the phase boundaries in each pressure-temperature region.

9. The method for calculating the condensate gas pressure-temperature phase diagram according to claim 8, characterized in that: The area described in step S1 includes three areas. When the saturated pressure is calculated, the temperature range of area 1 is 0-Tpmax, where Tpmax is the temperature corresponding to the maximum pressure point Pmax, and the temperature ranges of areas 2 and 3 are both Tpmax-Tmax.

10. The method for calculating the condensate gas pressure-temperature phase diagram according to claim 9, characterized in that: The initial calculated pressure range of the region 1 is Ptmax-70MPa, the initial calculated pressure range of the region 2 is Ptmax-70MPa, and the initial calculated pressure range of the region 3 is 0.1MPa-Ptmax.

11. Application of the condensate gas pressure-temperature phase diagram calculation method according to any one of claims 1 to 10 in the development of condensate gas reservoirs.

Images